diff --git a/express/execution-context-schema.ts b/express/execution-context-schema.ts
new file mode 100644
index 0000000..d191be3
--- /dev/null
+++ b/express/execution-context-schema.ts
@@ -0,0 +1,11 @@
+import { z } from "zod";
+
+export const executionContextSchema = z.object({
+    diachron_root: z.string(),
+});
+
+export type ExecutionContext = z.infer<typeof executionContextSchema>;
+
+export function parseExecutionContext(env: Record<string, string | undefined>): ExecutionContext {
+    return executionContextSchema.parse(env);
+}
diff --git a/express/execution-context.spec.ts b/express/execution-context.spec.ts
new file mode 100644
index 0000000..3a6f005
--- /dev/null
+++ b/express/execution-context.spec.ts
@@ -0,0 +1,35 @@
+import { describe, it } from "node:test";
+import assert from "node:assert/strict";
+import { ZodError } from "zod";
+
+import { parseExecutionContext, executionContextSchema } from "./execution-context-schema";
+
+describe("parseExecutionContext", () => {
+    it("parses valid executionContext with diachron_root", () => {
+        const env = { diachron_root: "/some/path" };
+        const result = parseExecutionContext(env);
+        assert.deepEqual(result, { diachron_root: "/some/path" });
+    });
+
+    it("throws ZodError when diachron_root is missing", () => {
+        const env = {};
+        assert.throws(() => parseExecutionContext(env), ZodError);
+    });
+
+    it("strips extra fields not in schema", () => {
+        const env = {
+            diachron_root: "/some/path",
+            EXTRA_VAR: "should be stripped",
+        };
+        const result = parseExecutionContext(env);
+        assert.deepEqual(result, { diachron_root: "/some/path" });
+        assert.equal("EXTRA_VAR" in result, false);
+    });
+});
+
+describe("executionContextSchema", () => {
+    it("requires diachron_root to be a string", () => {
+        const result = executionContextSchema.safeParse({ diachron_root: 123 });
+        assert.equal(result.success, false);
+    });
+});
diff --git a/express/execution-context.ts b/express/execution-context.ts
new file mode 100644
index 0000000..67f23e0
--- /dev/null
+++ b/express/execution-context.ts
@@ -0,0 +1,5 @@
+import { parseExecutionContext } from "./execution-context-schema";
+
+const executionContext = parseExecutionContext(process.env);
+
+export { executionContext };
diff --git a/master/master b/master/master
index 2ffc270..85f88a5 100755
--- a/master/master
+++ b/master/master
@@ -4,4 +4,6 @@ DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 
 cd "$DIR"
 
+export diachron_root="$DIR/.."
+
 ./master-bin "$@"